Microstrip antennas have come into widespread application because of the compact size and ease of fabrication. The conventional microstrip antenna consists of a rectangular patch metal element positioned on a grounded dielectric substrate. The thickness of the substrate is typically much less than the wavelength at which the antenna operates. Microstrip antennas are particularly desirable for use in an antenna array. Microstrip antennas, for example, are shown in U.S. Pat. Nos. 4,835,538 to McKenna, 4,131,893 to Munson et al., 4,131,894 to Schiavone, and 4,821,040 to Johnson et al. A disadvantage of a typical microstrip antenna is its narrow bandwidth, typically 3% and low gain, such as 7.0 db. It would be desirable to maintain the advantages of a microstrip antenna while improving its bandwidth and gain.
A number of approaches have been made to improve the bandwidth of microstrip patch antennas, but little attention has been paid to improving the radiation characteristics, such as directivity and gain. A number of approaches have been made to broaden the antenna bandwidth of microstrip antennas. These are a thick dielectric substrate microstrip patch and a multi-layer parasitically coupled microstrip patch antenna.
A thick dielectric substrate microstrip patch antenna such as shown in U.S. Pat. No. 4,835,538 as FIG. 1 comprises a radiating patch fabricated on a relatively thick dielectric substrate. Such an antenna structure can produce a bandwidth of approximately 8% at 1.5:1 VSWR (voltage standing wave ratio).
One approach to improving the bandwidth of a microstrip patch antenna is a design in which one or more parasitic elements are employed to improve the antenna bandwidth. An example of such an antenna structure is a capacitively coupled resonator radiator shown in U.S. Pat. No. 4,835,538 as FIG. 2. This includes a stacked array of two elements with only the lowermost element being fed. RF (radio frequency) energy is radiated from the driven element to create currents that flow on the parasitic element, which is larger than the driven element. This antenna structure produces a maximum bandwidth of approximately 14% at 2:1 VSWR. This is insufficient in many applications. Further, the VSWR obtained in this design is too high for the output stages of many RF transceivers and this can result in system inefficiency due to excessive return loss.
A further example of a multi-layer parasitically coupled microstrip patch antenna is also shown in U.S. Pat. No. 4,835,528 as FIG. 4. This antenna includes a stacked array of three circular elements in which the lowermost element is fed. The lowermost element is the smallest and the upper parasitic elements are the largest. These elements are printed on copper clad printed circuit board and are separated and supported by honeycomb dielectric material. The bandwidth obtained from this type of antenna structure ranges from 20-30% at 2.0:1 VSWR or about 18% at 1.4:1 VSWR. This bandwidth is broader as compared to conventional microstrip patch antennas, but, this antenna structure has a dual linearally polarized radiation characteristic. As a result, the RF energy is radiated in both the vertical and horizontal polarizations and this is not applicable or suitable in many applications, such as radio communication systems, which use vertical polarization only.
An antenna structure which has stacked radiator elements is shown in U.S. Pat. No. 4,131,892 to Munson et al.
A microstrip antenna and array of microstrip antennas is described in U.S. Pat. No. Re. 29,911 to Munson.
In view of the above state of development for microstrip antennas and the requirements for antenna applications, such as radio communications for cellular telephones, there is a need for an antenna, and corresponding array of antennas, which has a substantial bandwidth, high radiation efficiency, a reproducible design for easy manufacture and high power handling capability. There is further a need to control the radiation sidelobes for an array of such antennas.